Looking for a plastic/ceramic material USDA approved.

I have been looking for hours tring to find a material with these properties.
Working Temperature up to 500° F
Low coefiecient of Friction
Chemical Resistance to Costic Cleaners (mainly chlorinated)
Coeffecient of Thermal Expansion around 9.5 μin/in°F
decent machine ability
made in 1/4" flat

The use:
it will bolted to a piece of stainless steel and act as a air seal while a moving piece of stainless is ran along the length of it. The force between the oposing conpents is not great just enough pressure to keep them touching.

So you want a sliding sleeve which can maintain a pressure seal (how much pressure?) Will there be any lubrication? 500 F = 260 C, so not very hot. You could probably use a teflon tube if the air seal will hold.

There are many suppliers, and any experienced machinist can work with it - if they follow the directions! Requires sharp (new) carbide tooling, and obey the speed rules. Or you may be able to order the tube that you need. The coefficient of expansion is much lower than your specification.

Rather than look for something that meets all of your requirements, I would start with a vendor that has a good catalog of materials and sort down the list one property at a time, and see which is the most restrictive.

If you really want to maintain the air seal then the thermal expansion may be the critical issue. As an alternative why not use a bellows made from SS316?

Attached is the application, the friction or need of lubrication between the plastic/ceramic and the stainless is not nessecary also the movment is very slow maybe 6.0ft/min. We used Ultem and it is failing due to the chemical concentration of chlorine solution used to clean the machine. We thought about teflon but the cost is high. The length of this unit is 160' x 2 (both sides). Also the max temp is 360°F of this unit, and that is only at start up it drops down lower once all sections are running stable. The other big obsticle I am running into is that it needs to be Food Grade.

May be you could consider a nickel-Teflon composite coating, although probably it will not be cheaper than Teflon itself. There are also other low friction composites like the ceramic-teflon coatings used on frying pans but they are not so plastic. If you could use additional lubrication it would be easier for you to find an appropriate material.